Process of electrodepositing a manganese dioxide compound



' Aug. 5, 1947. R. F. CLEMENS PROCESS OF ELECTRODEPOSITING A MANGANESE DIOXIDE COMPOUND GRINDING MILL REDUCTION FURNACE k Mn PRODUCT STORAGE E 6 T 4 A R J U 8 ll E w 5 F L 8W \1 E E v P 5 W 8 4 W N S o A E 4 m 5 .R 5 I 5 MD F P E 4 D 4 VIE l m LE T E OF 1 V Y K m i w w A CH R L ,C E E S E E E L E R M m I N M :6 m 2 2 ER T U so k b I P f A E E O M R 2 "m 7 iln N G m M a vo RS I R WT V M ES F D N {I V E II E FR UR ME G A P DT MA. L P F IS 0 CM E SA vkflL NMR MW AE O 7 m BT. 3 2 3 5 A 3 3 M .l 3 3 3 ATTORNEY Patented Aug. 5, 194? raoosss OFELECTR'ODEPOSITING A MANGANESE DIOXIDE ooMroUNp Robert F. Clemens, New York, N. Y., assignor to The Dorr Company, New-York, N. Y., a corporation of Delaware Application August 31, 194%,Serial No. 500,611

1 Claim.

This invention relates to a process'of electrodepositing a manganese oxygen'containing compound. More particularly, it has to do with the depositing anodically of such compound and'in a substantially pur state;

'The manganese oxygen containing compound, commonly known as manganese dioxide, M1162, is in great demand as a depolarizer material for use in dry cell batteries. Such compound has heretofore been obtained by suitable treatment of a'manganese bearing ore.

Of the available manganese bearing'ores, there is the are known as pyrolusite, in which the manganese is primarily present in the form of an acid insoluble oxide admixed or associated with such objectionable constituents as iron oxides and silica. Another of these ores, is rhodochrosite, in which the manganese is present in the form of the acid soluble manganese carbonate but having associated therewith the objectionable constituents as iron and silica. While these ores carry a manganese content sufiiciently high for commercial exploitation, it has not been possible bythe treatments heretofore proposed to obtain a manganese product'of suflicient purity to maintain an overall degree of efiiciency as high as when employing this electrolytic manganese'prodnot as a depolarizer material.

Accordingly,'an object of the present invention is to provide aprocess'whereby a manganese product is produced which exhibits the overall high efficiency required for a satisfactory depolarizer material.

Another object of the present invention is'to provide a process of electrodepositing whereby an anodically manganese oxygen containing compound is produced which is substantially devoid of iron contamination.

Still another object of the present invention is to provide a process of electrodepositing a manganese containing compound anodically in which the initial source of the manganese is a manganese bearing ore having the contained manganese present in either acid insoluble or soluble form.

A further object of the present invention is to provide a process of electrodepositin'g a man ganese oxygen containing compound anodically in which-a manganese'bearingoreis employedas the initial source of the manganese.

A further object of the present invention is to provide a process of electrodepositing which will produce an anodic manganese oxygen-containing compound of a high degree of purity in a commercially practical and efiicient manner, and at aminimum cost.

Other features of novelty and objects will be apparent from the following description taken in connection with the accompanying drawing, wherein the single figure is adiagrammatic illustration of asystem for carrying out the treatment accor'di'ng to the present invention.

Referring to the drawing, manganese bearing orec'on'taining iron as an impurity and having the manganese present as an acid insoluble compound, like pyrolusite ore, ground or otherwise reduced 'to particles of suitable size, for example 8' mesh size, in the hopper IE3, is introduced through the pipe H into a roasting furnace l2, wherein "it is treated to convert such manganese compound into an acid soluble form. Pyrolusite oreha'sthe manganese present as manganese dioxide, MnOz, whichis insoluble in acid. To con vert the insoluble manganese dioxide, M1102, in the or into the acid soluble manganous oxide, MnO, the ore in the furnace i2 is subjected to a reducing roast. To do this city gas consisting chiefly "of hydrogen, methane, and carbon monoxide, is intro'duced into the furnace through the line '53 in an amount sufiicient to create a reducing'atmosphere therein, and such furnace is then heated to "a temperature of between 650 C. and 815 0., preferably 704 C., for about an hour or less. 'If desired, in place of roasting 'the ore in the presence of city gas, coal or other carbonaceous material in quantity suficient to produce reducing conditions may be incorporated withthe charge of ore andthe resulting-mass sub jected to the same roasting operation previously described. At the-completion of the roasting operation, the furnace is cooled to room temperature' while maintaining the roasted charge in the reducing atmosphere. When cooled to room temperature it-'is'stable and may be stored in contact with air without danger of re-oxidation of the manganous oxide to the acid insoluble manganes'e-dioxide.

The thus reduced ore is conveyed through the line It to a grinding mill l5 wherein it is ground I to'par-ticlesof a size-corresponding to approximately or mesh size. The thus ground roastedore 'may then be introduced through the line it into a leaching tank ll wherein it is subjected to an acid leaching operation to be subsequently described.

In place of the reduced pyrolusite ore, an ore likewis -containing iron as an impurity but having the manganese present as an acid soluble manganese compound, as for example rhodochrosite ore, rnay be employed. Rhodo'chrosite ore consists essentially of manganese carbonate, and the manganese component of such carbonate is mangan'ous oxide, MnO, which is acid soluble. Thus, the reducing roast treatment described in connection withlpyrolusite ore is not at all necessary iwithrhodochrosite ore. This last named ore, after being ground to a-suitable particle size of, for example 8 mesh size, is introduced into the hopper i8, and maybe discharged therefrom 3 directly into the leaching tank I1 through the line l9.

To the leaching tank H is introduced an acid leaching liquid in the form of spent electrolyte solution resulting from a cycle of cell operation,

it being supplied through the line 20 from the.

electrolytic cells designated by the numeral 2! by means of the pump 22. Such electrolyte solution contains manganous sulfate and free sulfuric acid. Water from the tank 23 is supplied to the spent electrolyte solution in the leaching tank through the line 24 in quantity such as to dilute it with almost enough water to make up for evaporation losses incurred during its period in the operation cycle of the electrolytic cells. The remainder of the Water to bring up the Water dilution of the electrolyte solution to fully compensate for evaporation losses suffered in the cell cycle is added as wash water at a later stage of the process hereinafter described. The spent electrolyte solution, after dilution, is analyzed for both manganous sulfate and free sulfuric acid content, and upon the basis of such analysis the required amount of either reduced pyrolusite ore or rhodochrosite ore to be added and acid, if any, is calculated. Should the calculations show that a quantity of dilute sulfuric acid should be added, such quantity of acid is supplied to the electrolyte solution in the tank I! through the line 25 from the tank 26.

The leaching tank is preferably made of wood, and is provided with a simple wooden paddle type agitator as indicated by the numeral 21, said agitator being so disposed that its peripheral ends are spaced only a slight distance from the bottom of the tank. The agitator is preferably driven at a speed of about 25 revolutions per minute. With the agitator thus arranged and operated at said speed, the solids, even of a particle size of 8 mesh, will be agitated just sufiicient to keep them in motion.

The spent electrolyte or leaching solution in the tank I! is heated to a temperature of 60 C. by direct injection of steam, whereupon the agitator 21 is caused to revolve. Either reduced pyrolusite ore from the grinding mill 15 or rhodochrosite ore from the hopper i8 is then supplied to the solution in quantity corresponding to the previously calculated amount. If pyrolusite ore is selected, it is supplied to the solution through the line l6. Should rhodochrosite ore be selected, it is supplied through the line l9. The agitation causes the ore particles to be brought into intimate contact with the leaching solution, and as a result of such contact the manganous oxide contained in the ore particles reacts with the free sulfuric acid to form more manganous sulfate. This reaction, exemplified by the following equation, is exothermic and causes a temperature rise of from to 10 C.

The reaction between the manganous oxide in the ore particles and the free sulfuric acid present in the solution continues until the acidity of the resulting ore pulp mass corresponds to a pH value of between 3 and l, whereupon it stops or is extremely slow, and for all practical purposes is complete. After agitation for about an hour, it is desirable to test the mass for its acidity. Should the test show that its acidity corresponds to a pH value of around 3, the leaching has proceeded satisfactorily. In the event that the test shows its acidity to be greater than that corresponding to a DH value of 3, a further addition of the ore in quantity sufficient to lower the acidity to a 4 pH value of around 3, preferably between 3 and 4, is introduced into the ore pulp mass. To insure the reaction of the maximum amount of the free sulfuric acid in the leaching solution with the manganous oxide of the ore particles, the ore is preferably added in quantity in slight excess over the calculated amount. Usually, an hour is suflicient for completion of the reaction. While the reaction rate and leaching yield of manganese will increase as the fineness of the ore particles is increased, it has been found that coarse ore particles, for example 8 mesh size, give a reasonable reaction time of about 1 hour and manganese extraction. yield close to Any other soluble constituents contained in the ore particles will also go into solution. One of such constituents present in the ore which readily goes into solution is iron, and since it has been found to be extremely detrimental, it is necessary that it be eliminated from the solution before the ore pulp mass is subjected to the filtering operation to be subsequently described.

In the leaching of the manganese ore thus described, the manganous sulfate removed from the electrolyte solution along with the generation of free sulfuric acid during its cycle in the electrolytic cells, is replenished by the reaction of the manganous oxide contained in the ore particles with such free acid. In a balanced system there is just sufficient free sulfuric acid in the spent electrolyte solution to bring the manganese concentration back to that desired for electrolytic cell feed. Practically, some fresh acid must be added to make up for mechanical losses and other acid consuming constituents in the ore.

Upon completion of the leaching operation, the ore pulp mass is withdrawn from the leaching tank I! through the line 28 and is conveyed through such line to the tank designated by the numeral 29, wherein it is subjected to a treatment for the separation or precipitation of the iron out of the manganous sulfate solution. To carry out such treatment, a basic substance capable of forming a precipitate upon reaction with the free sulfuric acid contained in the ore pulp mass, preferably inslurry form, is introduced into the tank 29 through the line 38 from the supply tank 3|, and such introduction continues until the acidity of the mass has been reduced to that corresponding to a pH value of between 5 and 6. The basic substance employed may be selected from the group consisting of calcium oxide, calcium hydroxide, calcium carbonate, sodium carbonate, barium oxide, barium hydroxide, barium carbonate, strontium oxide, strontium hydroxide, strontium carbonate, magnesium oxide, magnesium hydroxide, magnesium carbonate. Of the substances mentioned, it is preferred to use those of calcium. With the acidity reduced to the pH range of between 5 and 6, the mass is caused to undergo an oxidation by either subjecting it to an aeration'employing compressed air for a sufficient time or to the action of an oxidizing agent, for example manganese dioxide in the form of unroasted pyrolusite ore, in sufficient amount and time to convert all of the ferrous iron to the ferric state. For aeration, the necessary compressed air is supplied to the tank 29 through the line 32 from the receptacle 33. For the oxidation employing an oxidizing agent, the agent in the required amount is introduced into the tank 29 through the line 34 from the tank 35.

The treating of the leached ore pulp mass with the lime in slurry form in an amount such as to reduce its acidity corresponding to a pI-I value between 7 and 8 which is too close to the-pH range atwhich manganese is thrown out of solution to employ-it for the separation of the-iron in the ferrous state from the manganesesolution. The reactions of this treatment may. be expressed by the following equations:

With calcium hydroxide and aeration 4res0i 02 408(011); 2Hz0 4FB(OH)3 l, 4C3S04 L With calci1nnhydroxide and MnOz' I 2FeSO MnO2+ ononn 21320" With the completion of the addition ofthe-lime in slurry form to reduce the acidity of the leached ore pulpmass to a pH range-ofbetween 5' and 6,- it is desirable, before proceeding with the oxidation described above, to test asample of the-mass for iron content. Such test com prises the steps of filtering,'acidifying'thefiltrate, oxidizing the-acidified filtrate with permanganate, and then adding ammonium thiocyan-ate to the oxidized filtrate. Should" the thiocyacnate addition result in the formation of the characteristic red colo of ferric iron, there is ferrous iron in solution inthe ore pulp mass, Frequently there isenough unreduced MnOz in the reduced-pyrolusite ore particles to oxidize any iron present in the leached ore pulp mass.

From the tank 29, the 'ore pulp-mass is discharged through the line 36 into abatcn suction box or filter press t'l, in which it is subjected to vacuum filtration. By such filtration the manganous sulfate solution in substantially pure state and-devoid of iron passes through the suction box as the filtrate while the filter cake comprising the gangue, calcium sulfate and ferric hydroxide precipitates, and silica, is retained upon the box. Upon'completion of filtration, wash water from receptacle 38 is introduced through'theline 39 upon the suction box in quantity sufiicient to ex' tract most of the manganous sulfate solution that may be dispersed throughout'thefilter cake. By this washing of the cake the remainder of the waterto bring up the water dilution of the purified manganous sulfate solution filtrate to that required for an electrolyte solution cell feed, is introduced. The filter cake is withdrawn from the box 37 through the line llandais discharged to waste.

In place of discharging the ore pulp mass, upon completion of the iron separation treatment, into the filter press 3?,as previously described, the ore pulp mass may be permitted to settle over night in the tank 29, and then decanted and the sludge filtered through thesuction box or filter press and washed The decant liquor ispassed through the box to polish it prior to pumping'to the electrolyte cell feed tank 40.

The filtrate or manganous sulfate solution in a substantially pure state and devoid of iron is discharged from the suction box or filter press 3! through the line 42 and is conveyed-through such lineto-the electrolyte-cell feed'tank 40 by means of-pump 53. The tank- W is preferably made of wood and of a size suiiicient to hold enough electrolyte solution forone 24 'hourelec trolytic cell-cycle; From this tank-Ml, the electrolyte or manganous sulfate solution substantially devoid of iron is withdrawn through" line 44 and: is "conveyed; through such line continuously to the electrolytic cellsdesignated by. the 'nu-' meral 21;

The electrolytic: cells comprise rectangular shaped: wooden tanks=lined with lead, andihaving cathodes-of lead'and anodes of either lead or graphite suitably. suspended therein. At a point adj'acentthe feed end of the cells is disposed a vertical: U-sh'aped' lead coil through which steam is passed to heat and maintainthe cell ata temperature between 60' C. and 70 C., preferably 70 C; The'cell is of size such asto-accommodate 'ZO -anodes and 21"cathodes; E'ach anode has arr eifective'area of 14"square feet, andwith dimensionsof3feet' by. 2 /3 feet when submerged. The voltage'required' percell is between 2.5 and. 30 volts, and the optimum current densityfiis between'fi and" 9 'amperes per square foot-of anode surface.

A desirable: hook up of the electrolytic cells-is to arrange them in banks-or units of four with: the electrolytefeed being introduced into the first cell and'overfiowing into the second-cell, then into the third cell and so on, with the spent .electrolyte solution overflowing. from: the fourth cellbeing withdrawn and utilized for leaching the manganese bearing. ore; Electrically; the cells are hooked up-in a series-parallel arrangement to give amperage and voltage requirements that can be met with standard D. C. generatingequipment and which will 'notresult' in' excessive-bus bar sizes which is a function of the amperage alone:

In the electrolysis-of the-electrolyte solution of manganous'sulfate, manganese dioxide, MnOz, is deposited on the :anode and sulfuric acid equivalent to the managanese deposited is generated, while hydrogen: is evolved atthe cathode. The overall electrolysis reactionmaybe expressed by the following equation:

A-s-the exact electrochemical mechanism of this anodicmanganese dioxide deposit is not definitelyknown, it is to be understoodthat it is entirely diiferent tothat oi 'the usual electrodeposition of metals which are depositedon the cathode. Investigati'on' of: suchdeposit as regards acidity, specific gravity, and X-ray diffraction pattern, tends toindicate that it-is HzMnOs ratherthan MnOaHz'O, but as thishasnotyet been actually and definitely-determined it can not be stated as an' established fact.

The" electrochemical equivalent of manganese going-through the above valence change of 2 is 1.024 grams manganese per:- amper'ehour, which meansthat if of the currentimposed on a cell containing a' manganous sulfate electrolyte is utilized to efieot the above reaction, then every ampere hour of current usedwill result in the deposition of 1.024 grams-of manganese;

trical input to-produce manganese dioxide and some ofthe input is wastedsin merely, generating oxygen at theanode. The'ratio ofthe actual amount of. manganese deposited as manganese dioxide to that which should. be deposited, as computed from theabove' electrochemical :equivalent-iisv known asrthe current efficiency:

The current efiiciency: is'- affectedbyvariousfactors. It tends to decrease as the acidity in- Actuallyitiis notpossible to utilize 100% of the elec-- creases, although this effect is; not appreciable in the acid range of this process. Increasing the temperature tends to increase the current efiiciency and this is utilized by operating at 70 C., which is about as high as practical, due to evaporation. The presence of iron and reducing agents are known to reduce the current efficiency. The greatest single variable that can be controlled is the marked effect of anode current density on. current efiiciency. For example, as the current density is dropped from 18 amperes per square foot of anode surface to 3 amperes per square foot of anode surface, the current efficiency rises from 20% to 75%.

From the foregoing, it is apparent that the output of each cell will depend on the electrical input and the current eificiency. Since the current efficiency decreases as the input increases, it has been found that the actual cell output increases at a relatively low rate as the current input is increased. This is especially true in going from 6 amperes per square foot of anode surface to 12 amperes per square foot of anode surface, where it is found that the output is almost constant, due to the current efficiency falling off at a rate sufiicient to offset the increased current input. A current density of between 6 and 9 amperes per square foot of anode surface is, therefore, the optimum, Another factor to be considered in determining the optimum current density is the power requirements per unit of the product. As the current density goes up, the power requirements likewise go up, due to the current efficiency decreasing and to the higher voltage requirements at the higher current densities.

The deposit of manganese on the anode is removed periodically therefrom by lifting the anodes from the cells and taking off the deposit. With graphite anodes this is best done by means of a thin stainless steel spatula which can be inserted under the sheet deposit. Occasionally sections of deposit are difiicult to remove and it is best usually to let this deposit remain on the anode rather than damage same to get it off. The deposit on lead anodes can be cleaned oif best by tapping the deposit With a wooden mallet which causes it to break off rather easily. It appears that an operating period of six days between cleanings is about the correct interval.

The manganese anode deposit, after removal from the anodes, comprises pieces of a size having an overall width of from inch to 4 or inches and a thickness of about A; inch. Such deposit is introduced through the line 45 into a porcelain lined pebble mill 46. Water from the tank 47 is supplied to the mill through the line 48. While in the mill, the deposit is subjected to wet grinding until it has been reduced to about 95% minus 325 mesh size.

From the mill 46, the ground manganese deposit is discharged through the line 49 into the washing tank 50. Wash water from the tank 5| is supplied to the tank through the line 52. In the washing tank, the ground deposit is washed by successive batch re-pulpings, settling periods, and decantations, until the sulfate content of the decant liquor is the same as the wash water. On a large enough scale, continuous washing would be advantageous. The decant liquor is discharged from the tank through the line 53.

Upon completion of the washing, the manganese deposit is introduced through the line 54 into-the filter press 55, wherein it'is subjected to vacuum filtration. The press cake resulting from such filtration is discharged through the line 58 into the drier 57, and the filtrate is withdrawn through the line 58. In the drier, the press cake is subjected to a temperature of about 78 C. The dried cake is discharged through the line 69 into the storage tank 59 for subsequent disintegration and packaging as a finished product.

Various specific details of procedure and conditions of operation have been set forth for the purpose of giving a clear understanding of the present invention, but it is to be understood that the invention is not limited to the exact details given, as it includes modifications and changes coming within the scope of the appended claim.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

The process of electrodepositin a manganese dioxide compound which comprises treatin a leaching solutioncontaining manganous sulfate and free sulfuric acid with a manganese bearing ore containing iron and having the manganese present as an acid soluble compound in such amount as to neutralize said acid to a degree such that the acidity of said solution corresponds to a pH value of not lower than 3 and not more than 4 to thereby cause the acid soluble manganese compound in the ore to go into the leaching solution as manganous sulfate, treating the thus leached ore pulp mass with a basic substance capable of forming a precipitate upon reaction with said acid in amount such as to reduce its acidity to a pH value of between 5 and 6, subjecting said mass to an oxidation to convert the ferrous sulfate to the ferric state thereby causing the ferric iron to be precipitated out of said solution, separating the leaching solution from said mass to obtain a purified leaching solution substantially devoid of iron, passing a direct current through electrodes immersed in an electrolyte bath of said leaching solution While maintaining said bath at a temperature of not less than 60 C. and not greater than substantially 70 C. and at a current density not less than 6 and not more than 9 amperes per square foot of anode surface to deposit on the anode the manganese in the form of manganese dioxide, and forming said leaching solution by diluting spent electrolyte Withdrawn from said bath with Water and adding sulphuric acid as required to obtain the said acidity of said solution.

ROBERT F. CLEMENS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,167,700 Laist et al. Jan. 11, 1916 1,278,808 Cullen Sept. 10, 1918 1,322,000 Ellis Nov. 18, 1919 1,874,827 Storey Aug. 30, 1932 2,299,428 Rossetti Oct. 20, 1942 2,340,188 Jukkola Jan. 25, 1944 2,348,742 Mantell May 16, 1944 OTHER REFERENCES Transactions of the American Electrochemical Society, volume 33 (1918), pages 109-124.

Transactions of the American Electrochemical Society, volume .62 1932), pages 392-402. 

